Implementing RF power measurements in a broadband communications device

ABSTRACT

The invention provides systems, methods, and devices that compensate for temperature, frequency, and sampling effects in a broadband communication device&#39;s power measurements. One embodiment of a system includes a thermal device, and an automatic gain control circuit coupled to the thermal device. One method includes the acts of disabling a TOP operation, setting a RF input power, reading an AGC GAIN value, and setting the broadband communications device based on the read AGC value. Furthermore, a broadband communications device according to the invention may operate by disabling a TOP operation, setting a RF input power, reading an AGC GAIN value, and setting the broadband communications device based on the read AGC value.

RELATED APPLICATIONS

[0001] This disclosure is related to at least U.S. patent applicationSer. No. 60/165,725, filed Nov. 15, 1999, attorney docket numberTI-29915P, which is herein incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

[0002] This invention relates generally to broadband communications, andmore specifically, the invention relates to a method, system, andapparatus for implementing RF power measurements in a broadbandcommunications device, such as a DOCSIS compliant cable modem. Theinvention is applicable to at least those broadband devices comprisingthe LBTXXXXPHY chip family.

BACKGROUND OF THE INVENTION

[0003] The newer DOCSIS standards, such as ECO, recommend that BroadbandCommunication Devices (BCDs) have the ability to measure the power of aninput RF signal. Particularly, this feature recommended in DOCSIS 1.0and requires this feature in DOCKS 1.1. Measurement accuracy should bewithin 3 dBmV of real input power. Controller Modules (CMs) whichincorporate a low cost single conversion tuner in their front-endsolution suffer from a relatively large gain variance as the temperatureand frequency change. In fact, the overall gain variance may be as largeas +/−10 dB. This makes power measurement difficult.

[0004] One Broadband Communication device, the LBT403OPHY, uses a TakeOver Point (TOP) scheme for controlling AGC circuits in a CM. theinvention provides solutions that enable accurate power measurement in aBCD, using the qualities of the TOP scheme.

SUMMARY OF THE INVENTION

[0005] The invention provides technical advantages as systems, methods,and devices that compensate for temperature, frequency, and samplingeffects in a broadband communication device's power measurements. Oneembodiment of a system includes a thermal device, and an automatic gaincontrol circuit coupled to the thermal device such that the thermaldevice is enabled to compensate for variances in the automatic gaincontrol circuit.

[0006] In another embodiment, the method includes the acts of disablinga TOP operation, setting a RF input power, reading an AGC GAIN value,and setting the broadband communications device based on the read AGCvalue. Furthermore, a broadband communications device according to theinvention may operate by disabling a TOP operation, setting a RF inputpower, reading an AGC GAIN value, and setting the broadbandcommunications device based on the read AGC value.

DESCRIPTION OF THE DRAWINGS

[0007] The aforementioned features, and other features of the invention,will be apparent to those skilled in the art from the following detaileddescription. The detailed description is better understood, and shouldbe read in conjunction with, the accompanying drawings, in which:

[0008]FIG. 1 is an AGC interface amplification circuit with temperaturecompensation;

[0009]FIG. 2 shows an IF and RF AGC interface amplifier circuit withtemperature compensation;

[0010]FIG. 3 illustrate CVA8XMA IF and RF AGC gain response to controlvoltage at a single frequency;

[0011]FIG. 4 provides a graph illustrating IF/RF AGC control voltageresponse to a register value (across temperature);

[0012]FIG. 5 is a graph showing IF and RF gain reduction responses to aregister value, across selected temperatures (for a single frequency);

[0013]FIG. 6 depicts a graph showing IF and RF gain reduction responsewith TOP, to a register value, across temperature (for a singlefrequency); and

[0014]FIG. 7 provides a calibration algorithm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] A Take Over Point (TOP) is a separation point between a RadioFrequency (RF), Automatic Gain Control (AGC), and IF AGC control. If theTOP can be correlated well with a constant input power over a frequencyrange, a temperature range, and samples, a point exists within theDOCSIS defined input power range which divides the input power (Pin)into two sections:

[0016] 1. Pin<=TOP power; and

[0017] 2. Pin>TOP power at which the power is known.

[0018] The TOP power is attributed to an ‘AGC GAIN’ value in a broadbandcommunication device's (such as a LBT4030's) registers. Therefore, the‘AGC GAIN’ value at TOP represents a known power level. If a ‘GainReduction vs. AGC GAIN register value’ response is generated, the inputRF power from the AGC gain register value may be predicted. The spandivision by itself reduces the practical error by reducing the optionalinput power range for a given ‘AGC GAIN’ value.

[0019] It is helpful to consider that the division of an input powerspan influences different properties for each power range:

[0020] 1. Pin<=TOP power: this power range is frequency neutral, and theprimary contributors to variance are temperature and sample; and

[0021] 2. Pin>TOP power: this power range is frequency influenced. Bothtuner gain, and RF AGC GAIN reduction response vary with frequency aswell as temperature and power.

Maintaining Constant Top Power (Tempetature and Frequency)

[0022] One Solution for Lowering Temperature Effects of the RF/IF Pathon the TOP

[0023] Assuming the use of a temperature independent OperationalAmplifier (Op Amp) AGC interface circuit implemented in [a BCD] (such asa LBT403OPHY, 400D or 400E), the gain variance in the RF/IF signal pathacross different temperatures causes changes to the AGC GAIN register.In order to minimize the temperature effects on the system, atemperature compensation component should be included in the AGC loop.

[0024] A simple way to implement a temperature compensation component isto add a thermal device (RT) in an AGC interface circuit. The thermaldevice varies the gain in reverse polarity (inversely) to the IF/RF gainchange over temperature. For example, a thermistor device may be addedin parallel or series to a bias circuit such as RT 150 shown in FIG. 1(this setting is appropriate for a non-inverting AGC interface circuit.Other cases, such as an inverting case, require different RT 150placements, as is known in the art.)

[0025]FIG. 1 is an AGC interface amplification circuit 100 withtemperature compensation provided by a thermal device 150. The AGCinterface amplification circuit 100 includes a bias circuit portionhaving an operational amplifier 105, a first resistor R1 110, a secondresistor R2 120, a third resistor R3 130, and a fourth resistor R4 140,each of which is illustrated in FIG. 1.

[0026] When the RF/IF gain has an inverted relationship to thetemperature, with a non-inverting AGC polarity, a PTC (PositiveTemperature Coefficient) device may be appropriate for RT 150. Forexample, any gain reduction due to a temperature increase in turndecreases the negative bias. This increases the output voltage of theinterface circuit, thus keeping the AGC gain value constant (and thusderiving a constant TOP for a given input power).

[0027] This method of compensation may be used in any otheramplification circuitry, however, temperature response of otheramplification devices, such as Bi-polar transistors, should be takeninto account. This compensation may be added to an IF interface circuit,or to an IF and RF interface circuitry.

[0028] Alternative IF Circuit Compensation

[0029] Supplementing an IF section with a thermal device is less expensethan using a single thermal device. Because such compensation changesthe chain budget and may cause degradation (to NF on SFDR), the chainperformance of the tuner should be checked in all cases. Dualcompensation (IF and IR) may ensure a stable chain budget. However,because the IF and RF GC's have different responses, tuning gain overtemperature, requires non-linear circuitry.

[0030] The Thermal device RT 150 must have specific resistive qualities:ambient resistive accuracy, PTC/NTC, temperature coefficient, anddissipation constant. Ambient resistive accuracy should be about 1% aslarge as any resistor in the circuit, to lower gain errors. PTC/NTC issecured by using a RT having a positive coefficient. If using a NTC1device, it would be located in conjunction with R4 140. In addition, thetemperature coefficient of RT 150 has a curve matched to tuner's gainacross a temperature range. The tolerance of the device should beaccounted for, as well as a reasonable error. In many cases a linearapproximation may be made to the tuner's response. If an approximationis made, then the RT 150 should be linear as well. Furthermore, the RT'sdissipation constant (the power dissipated in RT 150) should either benegligible in the temperature budget, or calculated out.

[0031] Illustrative Device

[0032] For the following example, a P102GS1F (from U.S. SENSOR) uses anaccurate 1 kOhm resistor, with a near linear temperature response thatcorresponds with the near linear IF AGC GR response in the relevantspan. This example is based specifically on the LBT403OPHY referencedesign (LBT4030), using Toshiba's CVA8XMA single conversion tuner and anAGC interface circuit shown in FIG. 2. Accordingly, FIG. 2 shows an IFand RF AGC interface amplifier circuit with temperature compensation.Provided in the IF AGC interface 210 is a thermal device 215. Inaddition, the RF ACG 250 includes compensation considerations. Since thespecifics of the IF and RF ACG interfaces 210, 250 are discussed indetail in the available AN-LBT403OPHY-01 standard, which is incorporatedby reference herein (accordingly, details discussed in theAN-LBT403OPHY-01 standard are omitted from this discussion). However, itshould be understood that the same design principles and considerationsare applicable to many other specific embodiments.

TUNER PROPERTIES

[0033] Temperature Drift

[0034] Assuming a linear approximation in the tuner's gain drift acrosstemperatures of −0.08 dB/C (for ambient temperatures of about −10 toabout +60 degrees C.) the gain variance is about +/−2.8 dB relative to25 degrees C. (approximately room temperature). Actual designs shouldconsider the temperature inside the closed cable modem case.

[0035] AGC GAIN Response

[0036] The CVA8XMA's gain response for a single frequency is given inFIG. 3. Thus, FIG. 3 illustrates IF and RF AGC gain response for theCVA8XMA relative to control voltage (horizontal axis), at a singlefrequency. Gain reduction in FIG. 3 is illustrated on the vertical axis.

[0037] AGC Interface Circuit Properties

[0038]FIG. 4 “IF AGC control voltage” shows the response of the circuitshown in FIG. 2 to the EA register value, across temperature. Thehorizontal axis provides the Sigma Delta register value. The outputvoltage from the interface circuit (vertical axis) controls the IF AGCgain in a manner that compensates for the tuner's gain drift acrosstemperature, thus maintaining a stable AGC gain reading from theLBT4030.

[0039] Overall AGC Response With Temperature Compensation

[0040]FIG. 5 is a graph showing IF and RF gain reduction responses to aregister value, across selected temperatures (for a single frequency).The AGC response presented in FIG. 5 (“CVA8XMA IF/RF AGC GR response”)is an integration of the responses illustrated in FIG. 3 and FIG. 4. InFIG. 5, gain reduction is illustrated on the vertical axis, and AGC gainregister value is illustrated across the horizontal axis. Here, applyingthe TOP scheme will results in a span division.

[0041]FIG. 6 depicts a graph showing IF and RF gain reduction responsewith TOP=277, to a register value, across selected temperatures (for asingle frequency). Accordingly, FIG. 6 shows the overall gain reductionresponse of the tuner's IF and RF AGC due to the ‘AGC GAIN’ registervalue in the LBT4030, in conjunction with the interface circuitry. Gainreduction is illustrated on the vertical axis, while AGC gain registervalues are illustrated across the horizontal axis. The gain variance inthe IF AGC over temperature is approximately equal to +/−2.8 dB forabout a −15 to about a −3.1 dB gain reduction (which is more than thepractical IF span desired to be measured within the DOCSIS power range).As illustrated, values under −240 are not used in the scheme due to RFAGC limitations. This “spare” section is provided in order to cover theRF gain reduction across the input frequency range. It is probable toreceive a slope variation over frequency. In high frequencyapplications, the spare section is often used.

[0042] Alternative Solution

[0043] Another method of temperature compensation is to measure theambient temperature inside the CM with a digital readable device. Here,the TOP value power may vary with temperature. However, once thetemperature is known, the gain variation due to temperature drift may becalculated out, attributing the ‘AGC GAIN’ value to actual input powervariance as shown above. Typically, calibrations mentioned in theprimary mode are still valid for this solution as well.

[0044] Calibrating the TOP Over frequency

[0045] Another effect that shifts the TOP value is the gain response ofthe tuner over frequency. Frequency variance in the gain will lift theTOP for a given input power level. One method of keeping the TOP inputpower level constant over frequency is to prepare a calibration tableSW, setting the TOP as a function of the tuned frequency. Thecalibration table may consist of the full number of channels, or morepractically, frequency bands which share similar gain. Statisticalresponse of the tuner's gain versus frequency may help if the varianceacross samples is lower than the specified power measurement accuracy.In this case the calibration table across frequency may be the same oversamples, with the exception of a constant gain bias representing theabsolute gain variance over samples.

[0046] Method of Calibrating

[0047] Acquiring a TOP table requires a few acts that may be automatedto decrease a calibration time (for each modem). Automation may beaccomplished through a digital signal processor (DSP), a computerprogram, or other automation means. The procedure does not require anyintervention within the CM, and may be accomplished on a closed box atroom temperature.

[0048] Accordingly, FIG. 7 provides a calibration algorithm 700. thecalibration algorithm 700 begins with a disable act 710. In the disableact 710 the TOP operation of the LBT4030 is disabled by setting register0×3 to 0×2801 (which is the default value). Instead of disabling theTOP, the TOP may be set to the point to the lowest value (0×200) inregister 0×11, at which the TOP will never actually be reached. Next, ina set input act 720, the input RF power is set where TOP is required.Preferably, the input RF power is set from about −5 to about −9 dBmV.

[0049] Next, in a read AGC act 730, the AGC GAIN value on the LBT4030latched registers 0×A and 0×B is read. Then, in a repeat act 740, theset in put act 720 and the read AGC act 730 are repeated over thedesired frequency ranges. After the frequency ranges are read, in areturn act 750 the LBT4030 is returned to TOP operation, and the readAGC values are set as the TOP values for the appropriate tunedfrequencies.

[0050] Calibrating the TOP Over Samples

[0051] Statistical information of the tuner samples may reduce the needof sample calibration, if the variance is lower than the measurementaccuracy required. In most practical, low cost, single conversion tunersthis is not the case. Therefore, the need for calibrating each sample issubstantial. Thus, due in part to variance of the absolute gain and thefrequency response, a calibration for each sample is preferred.

[0052] Using the TOP Level for Power Measurement

[0053] Once the TOP power is established for a stable level accessfrequency, temperature, and samples, a conversion table for the ‘AGCGAIN’ values read from the LBT4030 may be constructed.

[0054] IF AGC Control Span

[0055] When Pin<=TOP power level, the IF AGC is active. Any ‘AGC GAIN’value>TOP value represents an input power lower than the TOP powerlevel. Defining the actual level may be done using the response given inFIG. 6, where each ‘AGC GAIN’ value is referred to a relative GR, whichmay be referred to as the TOP power. For example, the TOP was set to 277in the example above. Assuming this is equivalent to a RF input level of−5 dBmV, for an ‘AGC GAIN’ value of 300, a gain difference of about 8 dBrelative to TOP is obtained (which is translated to −13 dBmV).

[0056] IF A GC Calibration

[0057] The general procedure for obtaining the input power relation tothe ‘AGC GAIN’ values in the IF section would be to input the requiredpower span, and to record the ‘AGC GAIN’ with its corresponding powerlevel. This procedure will typically provide accurate measurements oversamples. However, because the IF AGC is not frequency dependant, theresponse may be predicted, and a curve definition may be derived. Thiseliminates the need to calibrate. In the example above and illustratedin FIG. 6, the IF GR response to ‘AGC GAIN’ values around the TOP may belinearly approximated resulting in a dB/LSB (dB per Least SignificantBit) number. In the example, the slope is approximately 0.33 [dB/LSB],which returns a (300-277)*0.33=7.67[dB].

[0058] RF AGC Control Span

[0059] When Pin>TOP power level, the RF AGC is generally active.Practically, any ‘AGC GAIN’ value<TOP value represents an input powerhigher than the TOP power level. Defining the actual level may be doneusing the response given in FIG. 6, where each ‘AGC GAIN’ value isreferred to a relative GR, that may be referred to as TOP power. Forexample, the TOP was set to 277 in the example above. Assuming that thisis equivalent to an RF input level of −5 dBmV, for an ‘AGC GAIN’ valueof 0 a gain difference of about 7.5 dB relative to TOP is obtained(which is translated to 2.5 dBmV).

[0060] RF AGC Calibration

[0061] The general procedure for obtaining the input power relation tothe ‘AGC GAIN’ values in a RF are similar to the IF. However, it must bedone over frequency (due to the GR variance of the RF AGC). Thefrequency dependence, and the RF AGC curve used, make it difficult toobtain an analytic approximation. Therefore, calibration over power ispreferred. The power versus ‘AGC GAIN’ values across frequencycalibration may be performed in conjunction with TOP calibration overfrequency. The procedure, per frequency, is to input the required powerspan and record the ‘AGC GAIN’ with its corresponding power level.

[0062] The input power increments are dependent on the AGC response andthe level of interpolation done between measured pints. For example, inFIG. 6, linear interpolation will require small power increments in the−100 range, and larger increments below −11, because of relative linearresponse. Another option is to use a logarithmic interpolation, whichrequires fewer measurement points.

[0063] Setting the ‘AGC GAIN’ Control Resolution

[0064] An important design related issue is setting the ‘AGC GAIN’control resolution. The control span for each of the AGC's (IF or RF) is1024 states (via a 10 bit register). When working in TOP mode, this spanis divided according to the TOP setting. The control resolution referredto in this section is the largest gain variation in dB per LSB. Thisresolution is determined by the GC slopes, interface gain circuit, andTOP setting. Low resolution will result in a large gain step per LSB.This should not affect the CM's performance so long as the controlvoltage after the LPF is much slower than a received symbol's duration.

[0065] ‘AGC GAIN’ Resolution Affect on Power Measurement

[0066] When measuring power, the gain step may cause a non-eligibleerror. The error may be up to one half of the maximum step value. In theexample above, the maximum step of the IF section is approximately 0.33dB/LSB for the practical range. This resolution may contribute an errorof to 0.165 dB in power measurement (which is typically reasonable). TheRE maximal step is about 0.33 dB/LSB maximum as well. Of course, in thesection of the AGC where the GR response is moderate, a much higherresolution is received (resulting in smaller errors).

[0067] Design Considerations for Minimizing Resolution Errors

[0068] Improving resolution reduces error introduced from a source.Accordingly, it is desired to improve resolution. Improving theresolution may be done by using an AGC with a moderate slope, using alow gain in the interface circuit, or by planning the TOP for maximumuse of the ‘AGC GAIN’ span.

[0069] Although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the followingclaims.

What is claimed is:
 1. A system that implements RF power measurements ina broadband communications device, comprising: a thermal device; and anautomatic gain control circuit coupled to the thermal device such thatthe thermal device is enabled to compensate for variances in theautomatic gain control circuit.
 2. The system of claim 1 wherein thethermal device is a variable thermister.
 3. The system of claim 1further comprising a temperature independent operational amplifier. 4.The system wherein the thermal device varies gain in reverse polarity toan IF/RF gain change across temperature.
 5. The system of claim 1wherein the thermal device has a positive temperature coefficientdevice.
 6. The system of claim 1 wherein the thermal device has anambient resistive accuracy of about 1 percent as large as any otherresistor in the circuit.
 7. The system of claim 1 wherein the broadbandcommunications device comprises a LBT4030 compliant device.
 8. Thesystem of claim 1 wherein the thermal device is a variable resistanceresistor.
 9. The system of claim 1 wherein the thermal device has atemperature coefficient that has a curve matched to a tuner's gainacross a temperature range.
 10. The system of claim 1 wherein thethermal device has a dissipation constant that is calculated based onthe resistance device coefficient.
 11. A method of calibrating abroadband communications device, comprising: disabling a TOP operation;setting a RF input power; reading an AGC GAIN value; and setting thebroadband communications device based on the read AGC value.
 12. Amethod of claim 11 further comprising the act of returning to TOPoperation.
 13. A method of claim 11 further comprising setting thebroadband communications device to a first predetermined frequency. 14.A method of claim 13 further comprising setting the broadbandcommunications device to a second predetermined frequency.
 15. A methodof claim 14 further comprising setting the RF input power at the secondpredetermined frequency.
 16. A method of claim 15 further comprisingreading a second AGC GAIN value based on the second predeterminedfrequency.
 17. A method of claim 16 further comprising setting the readAGC values as the TOP values for appropriate tuned frequencies.
 18. Abroadband communications device capable of being calibrated by:disabling a TOP operation; setting a RF input power; reading an AGC GAINvalue; and setting the broadband communications device based on the readAGC value.
 19. The broadband communications device of claim 18 furthercomprising setting the broadband communications device to a firstpredetermined frequency, and setting the broadband communication deviceto a second predetermined frequency.
 20. The broadband communicationsdevice of claim 19 further comprising setting the RF input power to thesecond predetermined frequency and reading a second AGC GAIN value basedon the second predetermined frequency.